A conventional bone conductive speaker has a known construction such as one shown in FIGS. 4(A) and 4(B). Such bone conductive speaker is constructed of: a cylindrical casing 21 provided with a diaphragm or vibrating plate 22 in its upper surface; a magnet 23 mounted in the casing 21 in an insertion manner; a yoke 25, which is mounted in a lower portion of the casing in an insertion manner and provided with a center magnet pole 24 in its central area; and, a voice coil 26, with which the center magnet pole 24 of the yoke 25 is encircled.
This type of conventional bone conductive speaker has its components such as the center magnet pole 24, the voice coil 26, the magnet 23 and the casing 21 concentrically arranged radially outwardly in the order of mention in the above. Due to this, and since both the center magnet pole 24 and the voice coil 26 are encircled with both the magnet 23 and the casing 21, it is necessary to increase both the magnet 23 and the casing 21 in diameter in order to increase the center magnet pole 24 in diameter and increase the number of turns for the voice coil 26. This makes it inevitably necessary to increase the bone conductive speaker in outer diameter.
In order to improve the bone conductive speaker in output efficiency without causing such increase in its outer diameter, a second type of conventional bone conductive speaker such as one shown in Japanese Patent No. 2967777 has been already proposed, which one is shown in FIGS. 5(A) and 5(B) and has a construction in which: a yoke 31, which is disposed in a housing 30, and provided with a center magnet pole 32 encircled with a voice coil 33; a magnet 34, which is disposed in each of a pair of diametrically-opposing ones 37 of four extension portions 37, 38 of the yoke 31; a pair of upright diaphragm-support portions 36 each disposed in each of a pair of the remaining extension portions 38 of the yoke 31; and, a diaphragm or vibrating plate 35 fixedly mounted on these diaphragm support portions 36.
This second type of conventional bone conductive speaker is called the outer magnet type, in which the magnets 34 are disposed in an outer peripheral portion of the speaker. In this second type of bone conductive speaker, the magnets 34 and the diaphragm support portions 36 (which corresponds to the casing 21 of the first conventional bone conductive speaker) are disposed on the same circle in contrast with the first conventional bone conductive speaker in which the magnets 23 and the casing 21 are concentrically arranged so as to form a doubled-wall structure. Due to this, the second type of conventional bone conductive speaker may be reduced in its installation space by half of its ancestor or first type of conventional bone conductive speaker. This makes it possible for the second type of conventional bone conductive speaker to be improved in output efficiency by increasing its center magnet pole 32 in diameter and increasing the number or turns for its voice coil 33, without causing any increase of its outer diameter.
In recent years, much more downsizing is required of the bone conductive speaker with the widening of its applications, for example such as incorporation in a cellular phone and like applications. In the outer magnet type of conventional bone conductive speaker, however, since its magnet is disposed in the outer peripheral portion of the speaker, and also since it is difficult to produce the magnet of a slim or thin-thickness type having a sufficient physical strength at a low cost, it is necessary for the outer magnet type of bone conductive speaker to prevent its magnet from being too reduced in size. Due to this, it is not possible for the conventional bone conductive speaker to sufficiently meet the demand for much more downsizing of the speaker.
Further another type of conventional bone conductive speaker is shown in FIG. 6. This type of conventional bone conductive speaker is called the inner magnet type, wherein: disposed in a central area of a bowl-like yoke 42 is a magnet 41 which is encircled with a voice coil 43; the yoke 42 is received in a housing 44; and, a diaphragm or vibrating plate 45 is supported on an upper surface of the housing 44.
In this type of conventional bone conductive speaker, however, it is necessary for the housing 44 to have a sufficient diameter in order to support the vibrating plate 45 in a manner such that the vibrating plate 45 is easily vibrated in operation. This is because the vibrating plate 45 is supported on the upper surface of the housing 44. Consequently, also in this inner magnet type of bone conductive speaker, it is not possible to meet the demand for much more downsizing of the speaker.
As described above, any type of the conventional bone conductive speaker fails to meet the demand for much more downsizing of the speaker in incorporating the bone conductive speaker into a cellular phone. Under such circumstances, the present invention was made to meet the demand. Consequently, it is an object of the present invention to provide a bone conductive speaker, which is easily produced at a low cost to meet the demand.